New surface treatment could improve refrigeration efficiency

Unlike water, liquid refrigerants and other fluids that have a
low surface tension tend to spread quickly into a sheet when they
come into contact with a surface. But for many industrial process
it would be better if the fluids formed droplets, which could
roll or fall off the surface and carry heat away with them.

Now, researchers at MIT have made significant progress in
promoting droplet formation and shedding in such fluids. This
approach could lead to efficiency improvements in many
large-scale industrial processes including refrigeration, thus
saving energy and reducing greenhouse gas emissions.

The new findings are described in the journal Joule,
in a paper by graduate student Karim Khalil, professor of
mechanical engineering Kripa Varanasi, professor of chemical
engineering and Associate Provost Karen Gleason, and four
others.

Over the years, Varanasi and his collaborators have made great
progress in improving the efficiency of condensation systems
that use water, such as the cooling systems used for
fossil-fuel or nuclear power generation. But other kinds of
fluids -- such as those used in refrigeration systems,
liquification, waste heat recovery, and distillation plants, or
materials such as methane in oil and gas liquifaction plants --
often have very low surface tension compared to water, meaning
that it is very hard to get them to form droplets on a surface.
Instead, they tend to spread out in a sheet, a property known
as wetting.

But when these sheets of liquid coat a surface, they provide an
insulating layer that inhibits heat transfer, and easy heat
transfer is crucial to making these processes work efficiently.
"If it forms a film, it becomes a barrier to heat transfer,"
Varanasi says. But that heat transfer is enhanced when the
liquid quickly forms droplets, which then coalesce and grow and
fall away under the force of gravity. Getting
low-surface-tension liquids to form droplets and shed them
easily has been a serious challenge.

In condensing systems that use water, the overall efficiency of
the process can be around 40 percent, but with
low-surface-tension fluids, the efficiency can be limited to
about 20 percent. Because these processes are so widespread in
industry, even a tiny improvement in that efficiency could lead
to dramatic savings in fuel, and therefore in greenhouse gas
emissions, Varanasi says.

By promoting droplet formation, he says, it's possible to
achieve a four- to eightfold improvement in heat transfer.
Because the condensation is just one part of a complex cycle,
that translates into an overall efficiency improvement of about
2 percent. That may not sound like much, but in these huge
industrial processes even a fraction of a percent improvement
is considered a major achievement with great potential impact.
"In this field, you're fighting for tenths of a percent,"
Khalil says.

Unlike the surface treatments Varanasi and his team have
developed for other kinds of fluids, which rely on a liquid
material held in place by a surface texture, in this case they
were able to accomplish the fluid-repelling effect using a very
thin solid coating -- less than a micron thick (one millionth
of a meter). That thinness is important, to ensure that the
coating itself doesn't contribute to blocking heat transfer,
Khalil explains.

The coating, made of a specially formulated polymer, is
deposited on the surface using a process called initiated
chemical vapor deposition (iCVD), in which the coating material
is vaporized and grafts onto the surface to be treated, such as
a metal pipe, to form a thin coating. This process was
developed at MIT by Gleason and is now widely used.

The authors optimized the iCVD process by tuning the grafting
of coating molecules onto the surface, in order to minimize the
pinning of condensing droplets and facilitate their easy
shedding. The process could be carried out on location in
industrial-scale equipment, and could be retrofitted into
existing installations to provide a boost in efficiency. The
process is "materials agnostic," Khalil says, and can be
applied on either flat surfaces or tubing made of stainless
steel, copper, titanium, or other metals commonly used in
evaporative heat-transfer processes that involve these
low-surface-tension fluids. "Whatever material you come up
with, it tends to be scalable with this process," he adds.

The net result is that on these surfaces, condensing fluids
such as liquid methane will readily form small droplets that
quickly fall off the surface, making room for more to form, and
in the process shedding heat from the metal to the droplets
that fall away. Without the coating, the fluid would spread out
over the whole surface and resist falling away, forming a kind
of heat-retaining blanket. But with it, "the heat transfer
improves by almost eight times," Khalil says.

One area where such coatings could play a useful role, Varanasi
says, is in organic Rankine cycle systems, which are widely
used for generating power from waste heat in a variety of
industrial processes. "These are inherently inefficient
systems," he says, "but this could make them more efficient."

The research was supported by the Shell-MIT Energy Initiative
partnership.

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